Relaying authentication is handled by the MTA, exim on cPanel servers, so you'd need to look at /etc/exim.conf which you should not edit directly (use the Exim Configuration Editor in WHM). As to what changes you might need to make, you'll have to trawl through www.exim.org

That said, making any such changes wouldn't be supported by cPanel. Why not use the inbuilt standard SMTP Authentication?